Lexus RX300 did not reverse?!

Hello! I own a 2001 Lexus RX300. I went to Starbucks this
morning, parked my car I felt the wheels hit the concrete
parking thingy on the ground however, everything was
fine. My car drove perfectly fine on my way to SB (just to
mention I recently got new rotors, brake pads and my
exhaust pipe repaired) so the vehicle is as quiet as mouse
and working in pristine condition. However, when I got
back into my car from getting my morning cup of joe my
vehicle did not back up after putting it in reverse. So I
gently placed my foot on gas pedal and it backed up but it
jerked hard, it made a knock noise and I was startled. I
immediately slammed my foot on the brake and it stopped
perfectly. I slowly put my car in Drive and drove back into
my parking space and tried to reverse again. It reversed
perfectly. My drive back home was perfect as well.
Backing up into my garage was smooth and perfect. This
incident scared me and I’d like to prevent it. Just by the
sounds of it. What do you think happened? Is it a serious
issue going on? Remember I just got it repaired so if’s my
brakes, tranny or engine my mechanic should let me know
since I’m paying to diagnose my car. Does this happen
often and I have nothing to worry about? Sorry if the
questions are silly. It’s my first vehicle and I’m new to the
car issues lol. Thanks I’m advanced for the answers!